Sen. Chris Murphy (D) of Connecticut, shown here on the Senate floor in a filibuster-like maneuver to force a vote on gun control, said he would stay there 'until we get some signal, some sign that we can come together.'

After 9/11, Democrats and Republicans stood on the steps of the US Capitol, held hands, and sang “God Bless America.” They created the Department of Homeland Security. They put air marshals on planes. They sent troops to Afghanistan.

After Sunday’s attack in Orlando, Fla., which was the largest terror-related attack in the US since 9/11, the House held a moment of silence for those killed and wounded. And then lawmakers went at each other. Republican Speaker Paul Ryan gaveled down shouting Democrats for being out of order with proposed gun legislation. Senate Democrats followed up on Wednesday by hijacking the floor with a nearly 15-hour talk-a-thon to force votes on gun control.

Admittedly, Florida is not on the scale of 9/11. But a tragedy such as Orlando “is a time when people rally around our country, and it’s obviously not what’s occurred, and it’s very disappointing,” said Republican Sen. Bob Corker, who is mentioned as a possible running mate for Mr. Trump. The senator’s home state of Tennessee last year suffered a drive-by shooting at a Navy Reserve center that killed five Americans.

Lethal mass-shootings have strafed America over the last two decades. Some, such as the Tennessee shooting, were related to terrorism. Some were not. In the local communities where they have occurred, citizens and leaders have moved toward healing. But not Washington. Lawmakers appear to be locked in an election-year standoff, immobilized by hot-button issues – though there are some signs of common ground.

“When a community sees its existence in jeopardy, as each of these communities do, that brings these communities together,” says Christopher Kojm, who served as deputy director of the bipartisan 9/11 Commission. “As a nation, we felt that on 9/11, but we haven’t felt that since.”

Why Orlando hasn't unified the nation

The Orlando attack, launched by a young American Muslim, Omar Mateen, who claimed allegiance to the Islamic State during the rampage, is “fraught with all kinds of division … because it had to do with guns, and it had to do with Muslims, and those are just going to be divisive,” says John Feehery, former spokesman to Republican Dennis Hastert, who was speaker of the House on Sept. 11, 2001. Democrats add the LGBT community to this list of politically polarizing topics.

At the same time, the Florida attack occurred when a high-stakes presidential election year is well under way (by contrast, 9/11 struck at the start of a new presidency). Not only have the two presumed presidential nominees and the president been battling over Orlando and terrorism, but Senate Democrats, who need only five seats to regain control (four, if they win the presidency), have seized the moment.

Sen. Chris Murphy (D) of Connecticut, which was rocked by the 2012 Newtown school shooting, said he would continue to hold the Senate floor "until we get some signal, some sign that we can come together."

That signal came when Republicans eventually agreed to allow votes on Democratic legislation to expand background checks to gun shows and online sales, and to prevent suspected terrorists from buying guns.

House Democrats are also pressing the issue. Congress hasn't significantly changed gun laws since 2007, after the Virginia Tech shooting, according to the Associated Press.

Sen. Chuck Schumer (D) of New York said of Republican senators on Tuesday, “Hopefully, they’ll change their minds out there. But if not, they will face the consequences and that makes progress move forward.”

The fact that Orlando-like attacks have occurred with unfortunate frequency also leaves the subject open to politicking. Most people can remember where they were when hijacked planes plowed into the World Trade Center, the Pentagon, and a Pennsylvania field on that clear September day15 years ago. It was an epic event in American history, like Pearl Harbor or the day that John F. Kennedy was shot.

“It’s sickening to say, but [mass shootings] are just part of the landscape, and this one was bigger, of course,” says Mr. Kojm, who is now a visiting professor at the Elliott School of International Affairs.

Singular events that threaten the entire country are galvanizing and unifying, he says. That explains why, on a smaller scale, local communities that have experienced severe tragedies – places such as San Bernardino, Calif; Charleston, SC; Fort Hood, Texas; or Newtown, Conn. – are motivated to come together to heal.

Observers such as Kojm and Feehery say that, sadly, it would take a bigger tragedy than Orlando to unify the political parties – or, more happily, policy areas of common ground, of which there are few. But they do exist.

Possible common ground on gun laws

On Wednesday, a House committee unanimously advanced a bipartisan mental health bill, which Republicans see as part of the answer to the complex problem of preventing mass shootings. On Tuesday, the Senate passed the annual defense spending bill, 85 to 13. Senate majority leader Mitch McConnell (R) of Kentucky referred to that as a “decisive step in the right direction” of helping to defeat the Islamic State.

Some Republicans are expressing interest in preventing suspect terrorists from buying guns, as proposed in legislation by Sen. Dianne Feinstein (D) of California. Senator Feinstein offered her bill last year in the wake of the shootings in San Berardino. It was defeated, along with an alternative bill from John Cornyn (R) of Texas. The two have talked about a compromise, but it looks like a rematch is again in the offing.

Interestingly, Trump – whose Orlando-related comments about banning Muslims and linking the president to terrorism have been denounced by some Republicans on the Hill – now says he’ll talk to the National Rifle Association about not allowing terrorists on the “watch list” or “no fly” list to buy guns.

The sticking point for Republicans – and the NRA – has been about due process for people wrongly on these lists, a point that Feinstein says is already addressed in her bill.

Thinking back to last year’s shooting in San Bernardino, Feinstein said that the city has “come together” since the massacre, which also involved self-radicalized Muslims.

“San Bernardino came together. All of it. In every way, shape, and form. And I think the population of America is coming together over” Orlando, she said in a brief interview Tuesday. “Whether all of the politicians do, I can’t say.”